Implement accessibility enhancements (issue #1217)

- Add new CefBrowserHost::SetAccessibilityState method for toggling
  accessibility state when readers are detected by the client.
- Add new CefAccessibilityHandler interface for the delivery of
  accessibility notifications to windowless (OSR) clients.
- Fix delivery of CefFocusHandler callbacks to windowless clients.
- cefclient: Add example windowless accessibility implementation on Windows and macOS.
- cefclient: Automatically detect screen readers on Windows and macOS.

libcef_dll/cpptoc/accessibility_handler_cpptoc.h

@@ -0,0 +1,34 @@
+// Copyright (c) 2017 The Chromium Embedded Framework Authors. All rights
+// reserved. Use of this source code is governed by a BSD-style license that
+// can be found in the LICENSE file.
+//
+// ---------------------------------------------------------------------------
+//
+// This file was generated by the CEF translator tool. If making changes by
+// hand only do so within the body of existing method and function
+// implementations. See the translator.README.txt file in the tools directory
+// for more information.
+//
+
+#ifndef CEF_LIBCEF_DLL_CPPTOC_ACCESSIBILITY_HANDLER_CPPTOC_H_
+#define CEF_LIBCEF_DLL_CPPTOC_ACCESSIBILITY_HANDLER_CPPTOC_H_
+#pragma once
+
+#if !defined(WRAPPING_CEF_SHARED)
+#error This file can be included wrapper-side only
+#endif
+
+#include "include/cef_accessibility_handler.h"
+#include "include/capi/cef_accessibility_handler_capi.h"
+#include "libcef_dll/cpptoc/cpptoc_ref_counted.h"
+
+// Wrap a C++ class with a C structure.
+// This class may be instantiated and accessed wrapper-side only.
+class CefAccessibilityHandlerCppToC
+    : public CefCppToCRefCounted<CefAccessibilityHandlerCppToC,
+        CefAccessibilityHandler, cef_accessibility_handler_t> {
+ public:
+  CefAccessibilityHandlerCppToC();
+};
+
+#endif  // CEF_LIBCEF_DLL_CPPTOC_ACCESSIBILITY_HANDLER_CPPTOC_H_